Trying to to use free 3 month Apple TV

Having recently purchased an iPad, I've been given three months Apple TV for free. Except when I go to use this, I'm in a never-ending loop of having to sign in, confirm my account, enter new numbers again and again. I've tried on my TV (through Roku) and on my iPad but whenever I select the "Start free 3 months" it asks me to sign in, even though I already have. Been trying for the last 30 minutes. Any help would be much appreciated.